Vitamin D was discovered in 1920, ending the long search for a way to cure rickets, a painful childhood bone disease. Within a decade, foods were fortified with vitamin D, and rickets became rare in the United States.

But solving the problem of rickets was only the beginning of research into vitamin D. New results suggest that vitamin D may have an important role in many aspects of human health — from fractures to prostate cancer, from cardiovascular disease to neuromuscular problems and diabetes.

Vitamin D is best known for its role in keeping bones healthy by increasing the absorption of calcium in the intestines. Insufficient amounts of vitamin D lead to low bone calcium, which increases the risk of osteoporosis and bone fractures.

If vitamin D did nothing more than protect bones, it would still be essential. But there's now evidence that it may do much more. In fact, many of the body's tissues contain protein receptors that latch on to vitamin D: the intestines, the prostate, the heart, blood vessels, muscles and endocrine glands. And work in progress suggests that good things happen when vitamin D binds to these receptors. The main requirement is to have enough vitamin D. But many Americans don't.

Vitamin D deficiencies are shockingly common in the United States. According to a 2010 report, in fact, about 60% of American adults have insufficient levels of vitamin D. Why?

Limited exposure to sunlight heads the list. Except during the short summer months, people who live far from the equator can't get enough ultraviolet energy from the sun to make the vitamin D they need. The same is true for people who spend most of their time indoors, or for those of us who avoid sunshine and use sunscreens to protect our skin from the harmful effects of ultraviolet radiation.

Vitamin D deficiencies were rare when most men rolled up their sleeves to work in sunny fields. But as work shifted from farms to offices, that changed.

Skin pigmentation can reduce vitamin D production in the skin, nonwhite populations are at particular risk.

People with certain intestinal disorders or kidney or liver diseases are less able to converr vitamin D to its active form.

Some medications reduce the availability or activity of vitamin D.

Advancing age — even in healthy people — is linked to an increased risk of vitamin D deficiency.

Numbers can never tell the whole story, but in this case, D-ficiences add up to a wide range of health concerns.

Osteoporosis and Fractures

Without enough vitamin D, the intestines cannot efficiently absorb calcium. But the body has a way to prevent blood levels of calcium from falling too low: It pours out parathyroid hormone, which draws calcium from bone. Blood calcium levels remain normal, so your heart and nerves keep working nicely. But your bones bear the brunt: As bone calcium density falls, bones become weak and prone to fractures.

Prostate cancer

Vitamin D has an important role in regulating cell growth. Laboratory experiments suggest that it helps prevent the out-of-control cell growth that characterizes cancer. Like many human tissues, the prostate has an abundant supply of vitamin D receptors. It's far too early to say if vitamin D can reduce the risk of prostate cancer, but it's a possibility that deserves additional study.

Other cancers

The risk of colon cancer, breast cancer and other cancers appears to rise in people who live far from the equator. Sun exposure and vitamin D levels may be part of the explanation. With prostate cancer, the evidence varies from study to study. A 2006 review of studies suggests that, overall, vitamin D is beneficial for reducing the risk of colon cancer. But a seven-year study of women who took 400 IU of vitamin D and 1,000 milligrams of calcium a day found no protective effect for colon cancer.

Neuromuscular function

Osteoporosis increases the risk of fractures. Falling also breaks bones. Vitamin D can help with both brittle bones and falls.

A 2004 analysis of five randomized clinical trials concluded that vitamin D supplements reduce the risk of falling by 22%; most of the participants were women. A 2006 study reported that a daily dose of 700 IU produced a 46% reduction in falls in women but no protection for men. The benefit was greatest in older, less active women. The possible difference between the men and women may be explained by differences in physical activity, muscle size or diet. Vitamin D binds to receptors in muscles, so improvements in strength and balance may account for the benefit.

Vitamin D deficiency has been linked to persistent and otherwise unexplained muscle and bone pain. For some patients get relief with vitamin D supplements. Several studies have linked a high consumption of vitamin D with a reduced risk of multiple sclerosis.

Heart and blood vessel disease

Vitamin D may play an important role in blood vessel health. People with low blood levels of vitamin D appear to have an increased risk of coronary artery calcifications (caclium deposits). Although animal studies suggest that vitamin D has a role in regulating blood pressure, a Harvard analysis found no link between vitamin D and high blood pressure (hypertension) in humans. Still, low levels of the vitamin appear related to an increased risk of stroke and heart failure.

Other disorders

Although experiments show that vitamin D can protect animals from diabetes, human studies are less convincing. A study of 87,779 women did not find an overall protective role, but it did suggest that high doses of vitamin D (more than 500 IU a day) and calcium (more than 1,200 milligrams a day) might reduce the risk of type 2 diabetes by 33%.

High levels of vitamin D have also been linked to better lung function, a stronger immune system, and an enhanced ability to fight off certain infections, such as tuberculosis. All these potential benefits require additional study, but vitamin D ointments have already been approved to treat psoriasis.

Until 1997, the recommended daily allowance (RDA) for vitamin D was 200 IU for all adults. Faced with growing evidence of vitamin D deficiencies in Americans, the RDA was increased to 400 IU for 51- to 70-year-olds, and to 600 IU for people older than 70.

Is more better? New research suggests that it is. Many experts now recommend 800 to 1,000 IU a day. Remember, though, that you can get too much of a good thing. Like the other fat-soluble vitamins, vitamin D is stored in the body's adipose (fat) tissue. This means that your body can draw on its own reserves if your daily intake slips temporarily. But it also means that excessive doses of vitamin D can build up to toxic levels. However it takes massive overdosing to produce problems, and amounts up to 2,000 IU a day are considered safe.

You can make your vitamin D the old-fashioned way — by exposing your skin to ultraviolet radiation in sunlight. But it's easy to overdose on sunlight, which increases the risk of malignant melanomas and other skin cancers, wrinkles and premature skin aging. All in all, most doctors recommend avoiding sunlight and getting vitamin D by mouth.

Diet can help, but it's very difficult to get the new recommended amount through food alone. Fish and shellfish provide natural vitamin D (oily fish are best), but you'll have to eat about 5 ounces of salmon a day to get just 400 IU. Other foods have even less D, which is why manufacturers fortify milk, some yogurt, some orange juice and many cereals with vitamin D. In general, a serving will provide about 100 IU; that means drinking a quart of fortified milk to get 400 IU.

Most people require supplements to get the vitamin D they need. Many formulations are available, but remember to read the labels carefully so you won't get too little or too much. And although cod liver oil is rich in vitamin D, it has too much vitamin A for regular use.

It used to be simple: Just get a “healthy” tan and your body will make all the vitamin D it needs. Desk jobs and sunscreens have changed all that. And now research is showing just how important vitamin D is and suggesting its possible role in preventing many health problems. That makes Vitamin D a d-lemma of modern life that has a modern solution: Eat fish and drink some low-fat fortified milk, and take a supplement with 800 IU to 1,000 IU of vitamin D a day.

Harvey B. Simon, M.D. is an Associate Professor of Medicine at Harvard Medical School and a member of the Health Sciences Technology Faculty at Massachusetts Institute of Technology. He is the founding editor of the Harvard Men's Health Watch newsletter and author of six consumer health books, including The Harvard Medical School Guide to Men's Health (Simon and Schuster, 2002) and The No Sweat Exercise Plan, Lose Weight, Get Healthy and Live Longer (McGraw-Hill, 2006). Dr. Simon practices at the Massachusetts General Hospital; he received the London Prize for Excellence in Teaching from Harvard and MIT.

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